1. Field of the Invention
This invention relates generally to a remote control system for controlling a plurality of functions such as channel selection, sound adjustment, luminance adjustment and so on of a television receiver or the like, and is directed more particularly to a system using a command signal including a series of alternating mark periods and space periods.
2. Description of the Prior Art
In the art, a remote control system for a television receiver or the like is known which uses an ultrasonic wave signal of, for example, 40 KH.sub.z. In such a system, when the ultrasonic wave signal is supplied once, the channel of the television receiver is switched, for example, by one. If the channel selection is controlled only by detecting whether the ultrasonic wave signal is present or not, it is very easy for erroneous operation to be caused by noise.
In order to avoid this erroneous operation one known system uses an ultrasonic wave signal transmitted intermittently at a predetermined frequency. The signal has a mark period and space period which are repeated. In the receiver, a control signal is produced only when an ultrasonic wave signal having a predetermined mark period is received repeatedly a predetermined number of times. Thus, erroneous operation caused by noise can be avoided.
It is also desirable that, in addition to channel selection, the same remote control system be used for the adjustment of sound volume and luminance. To this end, it is necessary to have a system capable of transmitting the number of instructions corresponding to the number of functions to be controlled. Therefore, it is known to control the intermittent frequency in response to the function selected, and, at the receiving end, to detect successive periods (mark periods) of the ultrasonic wave signal to carry out the various operations in accordance with the detected length of successive period (mark periods).
However, with the above multi-function system it is possible for a noise signal occurring within the space period to be detected and thereby cause the system to operate erroneously. To avoid this it has been suggested to only consider the ultrasonic wave signal present when it is detected continuously for more than 30 msec (milli-seconds) and when the signal is present less than 30 m sec it is considered to be noise.
In the above case, however, if the tolerance error for detecting the successive time periods (mark periods) is selected as, for example, .+-.10% and if a signal having a predetermined period is detected repeatedly four times to produce an output signal (control signal), the time necessary for detection is shown in the following table I in which A.sub.1, A.sub.2, A.sub.3, A.sub.4 and A.sub.5 represent control signals or command signals.
Table I ______________________________________ Mark Period Minimum -10% .about. t.sub.1 + 10% Space period Detecting Period (m sec) t.sub.2 (=t.sub.1) (m sec) t.sub.1 .times. 4 + t.sub.2 .times. 3 (m ______________________________________ sec) A.sub.1 30.6 .about. 34 .about. 37.4 34 238 A.sub.2 37.8 .about. 42 .about. 46.2 42 294 A.sub.3 46.8 .about. 52 .about. 57.2 52 364 A.sub.4 57.6 .about. 64 .about. 70.4 64 448 A.sub.5 71.1 .about. 79 .about. 86.9 79 553 ______________________________________
In the Table I, t.sub.1 represents the mark period and t.sub.2 the space period, respectively.
It is demonstrated from the above Table I that to control more than five functions with the above system could require more than 0.5 seconds to convey a command.